Production and release mechanism of N₂O in agricultural soils irrigated with acid mine drainage

Acid mine drainage (AMD) is mostly untreated or not up to standard level before directly drained into rivers for irrigation, causing severe pollution of agriculture eco-environments. Metal pollution had been widely reported in extensive fields including the red soil region in South China. As we have known, N₂O emitted from agricultural systems was one of the important causes of global greenhouse effects. However, there has been poor knowledge of potential changes in N₂O evolution in polluted fields. In this study, four agricultural soil profiles from sugarcane and paddy fields were used to track the changes in N₂O emission and sources of heavy metal polluted soils irrigated with AMD (Shangba Village, Wengyuan County, Guangdong Province) and then compared with unpolluted soils irrigated with natural water (Lianxin Village, Wengyuan County, Guangdong Province). The physical/chemical parameters and contents of heavy metals in the soils, N₂O concentration and stable nitrogen and oxygen isotope compositions were analyzed to determine the contribution of nitrification and denitrification of N₂O and the reduction ratio of N₂O. Our results showed that there was slightly higher N₂O concentration of the same crop in AMD irrigated area than in unpolluted soil irrigated with natural water, and higher soil N₂O concentration in sugarcane fields than in rice fields. The production of N₂O from denitrification was 71.29%, which was higher than that from nitrification in surface soil (0-30 cm) in sugarcane fields in areas irrigated with AMD. N₂O reduction ratio in the soil profile in AMD irrigation area decreased gradually with increasing depth. There was only 15.54% N₂O reduction to N₂ at the peak of N₂O concentration. However, the average ratio of N₂O reduction to N₂ in sugarcane fields irrigated with natural water was as high as 49.80%. Limited N₂O reduction led to high levels of N₂O residues in the soil. Studies showed that AMD irrigation changed the production and release of N₂O by changing pH, heavy metal content and moisture content of agricultural soils. N₂O production and reduction studies carried out using combined nitrogen and oxygen isotope compositions clarified potential risks of irrigated agricultural soils with AMD. This provided the scientific basis for future restoration works in polluted soils.